Liquid crystal displays are commonly used as display devices for compact electronic apparatuses. This is because they not only provide good quality images but also are very thin. Because liquid crystal molecules in a liquid crystal display do not emit any light themselves, the liquid crystal molecules have to be lit by a light source so as to clearly and sharply display text and images.
Referring to FIG. 4, a typical liquid crystal display 4 includes a liquid crystal panel 41 and a backlight module 42 adjacent to the liquid crystal panel 41. The backlight module 42 is configured for providing light beams for the liquid crystal panel 41.
The liquid crystal display 4 includes a color filter substrate 411, a thin film transistor (TFT) substrate 412 opposite to the color filter substrate 411, a liquid crystal layer 413 sandwiched between the color filter substrate 411 and the TFT substrate 412, a first polarizer 414 disposed on an outer surface of the color filter substrate 411 and a second polarizer 415 disposed on an outer surface of the TFT substrate 412. The first polarizer 414 has a first polarization direction. The second polarizer 415 has a second polarization direction perpendicular to the first polarization direction of the first polarizer 414.
The backlight module 42 includes a light source 421, a light guide plate 422 and a reflective plate 423. The light guide plate 422 includes a light incident surface 4221, a light emitting surface 4222 and a bottom surface 4223 opposite to the light emitting surface 4222. The light emitting surface 4222 and the bottom surface 4223 are both perpendicularly connected with the light incident surface 4221. The light source 421 is disposed adjacent to the light incident surface 4221 of the light guide plate 422. The reflective plate 423 is disposed adjacent to the bottom surface 4223 of the light guide plate 422.
Light beams emitted from the light source 421 enter the light guide plate 422 through the light incident surface 4221, and propagate in the light guide plate 422. A majority of the light beams are emitted out from the light emitting surface 4222 and are utilized by the liquid crystal panel 41. A minority of light beams transmit out from the bottom surface 4223, and reach the reflective plate 423. These light beams are reflected by the reflective plate 423 and transmit back into the light guide plate 422 again.
However, the second polarizer 415 has a second polarization direction to allow light beams having a polarization direction parallel to the second polarization direction to transmit through. The transmitted light beams are difined as P-ray. Other light beams having polarization directions which are not parallel to the second polarization direction of the second polarizer 415 can not transmit through the second polarizer 415. The light beams reflected by the reflective plate 423 are unpolarized and can be devided into 50% P-ray and 50% other rays. Only the P-ray has a polarization direction parallel to the second polarization direction of the second polarizer 415. Therefore, only 50% P-ray of the reflected light beams can transmit through the second polarizer 415 and be utilized. Thus, a utilizing efficiency of the reflected light beams is only 50%. The low light utilizing efficiency becomes a serious problem of the liquid crystal display 4.
What is needed, therefore, is a backlight module that can overcome the above-described deficiencies. What is also needed, is a liquid crystal display including such backlight module.